24 January 2011

Scanning salmon smelling streams

You could be forgiven for thinking that there’s no way you could get a useful brain scan of a fish.

First all, if you pay attention to brain scanning, you have probably heard a story about fish in fMRI machines that has become the stuff of scientific legend. The salmon in question being scanned for brain activity was deceased. Pushing up daisies. Size feet under. Joined the choir invisible. In a word, dead.

Yet a brain scan revealed statistically significant brain activity.

Bennett and colleagues were making a point that simple statistics on brain scans could be misleading. But they may have inadvertently dampened other salmon related scanning studies. Brain scans are increasingly being used on small animals. Here, brain scanning gets turned towards studying one of the oldest and deepest questions about salmon behaviour: their ability to return home.

Salmon, like sockeye salmon (Oncorhynchus nerka), are hatched in freshwater streams. From there, they swim out to the ocean, spend years growing and growing, then swim back to the same watershed they were hatched in. That ability to find that original stream, out of the hundreds or thousands pouring into the oceans baffled people for a long time.

Given the complexity of the task, there is probably going to be no single explanation for the salmon’s homing ability, but it seems like smell is playing a large role. There is both behavioural and physiological evidence supporting this.

It’s difficult to build up a cohesive picture of brain activity associated with something like smelling with traditional electrophysiology. You can only record a few cells in a brain at a time. This is where fMRI can shines: looking at the big picture.

You would predict would that the salmon’s brain would show some responses that would occur to pretty much any odour, and a different response to water from the stream a salmon was reared in when it was young.

The trick in an experiment like this is what do you use for a control? Fresh water was used as the “rest” baseline stimulus. To test something that had a definite smell that wouldn’t be associated with a stream, Bandoh and colleagues used amino acids. Amino acids are the building blocks of proteins, and lots of aquatic organisms respond strongly to amino acids in the water. Amino acids are probably the equivalent of the smell this gives off:

The stream water contains amino acids, too, but at concentrations about 20,000 times lower than the amino acid solution used as a control. The experimenters gave the fish fresh water for 10 minutes, an odour for 10 minutes – either the stream water or water laced with amino acids – and then back to non-smelly fresh water for 10 minutes.

The two fluids caused very similar brain activation in the olfactory bulb, which is is basically one step from the nose. If anything, the amino acids activate the bulb more strongly than the stream water.

But things started to differ as the signal went deeper into the brain, through more steps of processing.

When you got into the main part of the brain, the telencephalon, the stream water caused some areas of the brain to activate that the amino acids didn’t. And those regions that were activated by both, the response was stronger to stream water than the amino acid mix.

There are all sorts of interesting questions you could ask with these methods. Could you show that fish that get lost and don’t make it back to their home stream show a weaker signal than those that do? Could you show migrating species have this signal but not non-migrating species? While these could be answered with other techniques, it seems that fMRI might be able to do it a bit more efficiently.